Recently, an a-SiH film is in a development stage for a wide practical application such as a solar cell, an optical sensor, a field effect transistor (FET), an electrophotographic photoreceptor and etc, and is a remarkable material for electronics industry.
As methods for producing such a-SiH, there are known methods that a silicon target is sputtered in an atmosphere of active hydrogen, and that SiH.sub.4 or SiF.sub.4 gas is decomposed by glow discharging.
By sputtering method, there can be hardly obtained an a-SiH film having good characteristics because it is difficult to control a hydrogen content which have significant effects upon film characteristics of an a-SiH.
On the other hand, by glow discharging method, there can be obtained a film having less defects such as dangling bond, void, or the like. The glow discharging method also has a merit that control of valence electrons for n or p type a-SiH is possible, and that a film can be formed onto an appropriate shape of a substrate (i.e. a film having a wide area can be obtained). Therefore, recently, an a-SiH film having good characteristics is mainly produced by glow discharging method.
However, an a-SiH film produced by the conventional glow discharging method has an insufficient photosensitivity region, particularly at a long wavelength of longer than 700 nm. Therefore, for example, when such a film is an image pickup element, the film can not cover all regions of visible rays to pickup an image. When a film is used for a solar cell, such a film can not cover a long wavelength region of a sunlight. It induces a disadvantage that a sufficient cell efficiency can not be obtained because a light having a long wavelength region is not utilized. Further, when such a film is used for a electrophotographic photoreceptor, there are defects such as limitation of colors to be used, and the like. Moreover, when such a film is used for a photoreceptor in a semiconductor laser printer having a light source of semiconductor laser, it also has an insufficient photosensitivity for a practical use. Thus, an a-SiH film for a optical functional device has been insufficient in optical characteristics.
Then, there has been hitherto produced a silicon alloy containing metals such as germanium, tin and the like as an impurity in order to promote the absorption of a light having a low energy and to provide a photosensitivity having a long wavelength by making a band gap energy narrow. The introduction of such metals, however, makes disorder of an a-SiH structure itself and involves a defect of remarkable increase of a defect level. For example, when an a-SiH film is used as a photoreceptor, the increase of a defect level in an a-SiH film causes residual voltage to make a trouble of so called "memory, ghost image", so that it injuries a commercial value of an a-SiH film.
The above glow discharge method has a further problem to be solved, that is, for example, it has a low deposition rate or low forming rate of a film. The conventional glow discharge method only achieves 1-2 .mu./hr at maximum.